The familiar axisymmetric zones and belts that characterize Jupiter's weather system at lower latitudes give way to pervasive cyclonic activity at higher latitudes. Two-dimensional turbulence in combination with the Coriolis β-effect (that is, the large meridionally varying Coriolis force on the giant planets of the Solar System) produces alternating zonal flows. The zonal flows weaken with rising latitude so that a transition between equatorial jets and polar turbulence on Jupiter can occur. Simulations with shallow-water models of giant planets support this transition by producing both alternating flows near the equator and circumpolar cyclones near the poles. Jovian polar regions are not visible from Earth owing to Jupiter's low axial tilt, and were poorly characterized by previous missions because the trajectories of these missions did not venture far from Jupiter's equatorial plane. Here we report that visible and infrared images obtained from above each pole by the Juno spacecraft during its first five orbits reveal persistent polygonal patterns of large cyclones. In the north, eight circumpolar cyclones are observed about a single polar cyclone; in the south, one polar cyclone is encircled by five circumpolar cyclones. Cyclonic circulation is established via time-lapse imagery obtained over intervals ranging from 20 minutes to 4 hours. Although migration of cyclones towards the pole might be expected as a consequence of the Coriolis β-effect, by which cyclonic vortices naturally drift towards the rotational pole, the configuration of the cyclones is without precedent on other planets (including Saturn's polar hexagonal features). The manner in which the cyclones persist without merging and the process by which they evolve to their current configuration are unknown.
Giant planet tropospheres lack a solid, frictional bottom boundary. The troposphere instead smoothly transitions to a denser fluid interior below. However, Saturn exhibits a hot, symmetric cyclone centered directly on each pole, bearing many similarities to terrestrial hurricanes. Transient cyclonic features are observed at Neptune's South Pole as well. The wind-induced surface heat exchange mechanism for tropical cyclones on Earth requires energy flux from a surface, so another mechanism must be responsible for the polar accumulation of cyclonic vorticity on giant planets. Here it is argued that the vortical hot tower mechanism, claimed by Montgomery et al. and others to be essential for tropical cyclone formation, is the key ingredient responsible for Saturn's polar vortices. A 2.5-layer polar shallow-water model, introduced by O'Neill et al., is employed and described in detail. The authors first explore freely evolving behavior and then forceddissipative behavior. It is demonstrated that local, intense vertical mass fluxes, representing baroclinic moist convective thunderstorms, can become vertically aligned and accumulate cyclonic vorticity at the pole. A scaling is found for the energy density of the model as a function of control parameters. Here it is shown that, for a fixed planetary radius and deformation radius, total energy density is the primary predictor of whether a strong polar vortex forms. Further, multiple very weak jets are formed in simulations that are not conducive to polar cyclones.
Observations of the diurnal cycle in tropical cyclones (TCs) systematically indicate a ∼12‐hr offset between peak rainfall rate and the maximum height of anvil clouds in the TC cloud canopy. This phasing conflicts with archetypal models of organized deep convection, which suggest a tight coupling between rainfall, vertical cloud growth, and anvil clouds. We show that this phasing owes to the bimodal diurnal evolution of the transverse circulation, which peaks nocturnally from low–midlevels, and during daytime in the upper troposphere. The bottom‐heavy nocturnal circulation state is driven by latent heating from nocturnally invigorated deep convection, while the top‐heavy daytime state is the thermally direct circulation response to strong shortwave‐cloud warming in the optically thick TC cloud canopy. This daytime upper‐level circulation response manifests in a lifting of the maximum height of the TC outflow and, in turn, a lifting and invigoration of the upper‐level anvil clouds of the TC cloud canopy.
A recent observational analysis has reported significant repeating diurnal signals propagating outward at cloud-top height from tropical cyclone centers. Modeling studies suggest that the visible upper-level impacts reflect a diurnal cycle through the depth of the troposphere. In this study, the possibility of wavelike diurnal responses in tropical cyclones is characterized using 3D cloud-resolving numerical simulations with and without a diurnal cycle. Diurnal waves can only begin to propagate well beyond the storm core, and the outflow region is most receptive to near-core diurnal propagation because of its anticyclonic flow. The tropical cyclone structure appears particularly hostile to diurnal wave propagation during periods when the eyewall experiences a temporary breakdown similar to an eyewall replacement cycle.
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